Key telephone systems, of course, are well-known in the art. In such systems, plurality of lines have appearances at one or more stations so that each station may have access to more than one line. Audible and visual signalling is used to indicate the condition of each line, i.e., being rung, in use, off-hook, or being held. The signalling is applied to each line at all stations at which that line has an appearance.
Such systems invariably have an intercom whereby one station can communicate with another station by signalling the other station audibly, and whereby both stations may converse with one another over an intercom link circuit. An example of such an apparatus is shown in U.S. Pat. No. 3,787,640 to Bush et al. issued Jan. 22, 1974. In that patent, a non-locking key is used to implement and control the intercom feature.